This invention relates to a method of deactivating electromagnetic detection labels comprising a resonant circuit, in which an interrogation field is generated, the frequency of which is varied through a frequency range comprising the resonant frequency of the resonant circuit of the detection label, and in which a label is deactivated with an amplified interrogation field. The invention further relates to apparatus for deactivating electromagnetic detection labels comprising a resonant circuit, which apparatus comprises a transmitter/receiver with an antenna for generating an interrogation field, and means for generating a field amplified to a deactivating level. The invention also relates to an electromagnetic detection system comprising at least one detection zone in which, in operation, by means of one or more transmitters/receivers, an electromagnetic interrogation field can be generated for detecting detection labels comprising a resonant circuit, and a plurality of deactivating devices in which, in operation, detection labels can be detected and deactivated by means of a transmitter/receiver.
Electromagnetic detection labels, sometimes referred to as wafers or detection platelets, can be used in many situations for detecting the presence, and often also the identity, of a person, animal, vehicle, article, etc., in a detection zone. An important use for such detection labels is in shop-lifting detection systems. In such an application, each article to be protected is provided with a detection label which comprises a resonant circuit. Detection zones are formed near the exits of the shops, where an electromagnetic a.c. field, sometimes referred to as an interrogation field, is generated with the resonant frequency of the electromagnetic labels. Often, use is made of a sweep field, i.e., an interrogation field whose frequency varies periodically at a pre-determined rate between an upper and a lower limit. The resonant frequency of the labels is then intermediate these limits. As soon as a label is in a detection zone, the resonant circuit of the label is brought into the resonant state by the electromagnetic field. This fact can be detected either on the basis of the energy absorption caused by it, or on the basis of the secondary field formed by the label proper.
The labels are normally removed by a shop assistant at the cash desk, as soon as the protected goods have been paid for. In that case the labels do not reach the detection zone. If, however, it is attempted to take the goods outside without paying for them, the labels, which are mostly attached to the goods in a special way, are not removed. Such unremoved labels are detected in the detection zone, whereafter a signal can be given which reminds the customer of his obligation to pay.
The labels removed by the shop assistant at the cash desk are often designed for re-use. Alternatively, labels are sometimes designed to be used once only. Such labels could be removed at the cash desk, or could simply be deactivated, i.e. modified so that the deactivated labels can be carried through a detection zone without being detected. Deactivation should preferably be effected in a contactless manner, which offers the possibility of attaching the detection labels at a place which is difficult of access. Furthermore, a contactless and preferably also automatic deactivation promotes fast handling at the cash desk.
Such deactivatable labels can take the form, for example, of stickers.
In order that deactivatable labels may actually be deactivated, it is necessary for the characteristics of the resonant circuits of the labels to be modified in such a manner that the labels can no longer be detected. Known possibilities therefor are, for example, detuning the resonant frequency of the circuit to outside the detection range; changing the quality factor Q of the circuit to a low value; interrupting the circuit, or short-circuiting the circuit. To effect the change in characteristics, mostly one of the following two principles is used:
1. Dielectric breakdown in the capacitor of the circuit, resulting in a permanent short-circuit or decrease in quality factor;
2. The permanent interruption of the circuit by causing a fuse included in the circuit to blow.
Method 1 requires a high voltage to be generated across the capacitor. In method 2, however, a high current must flow through the circuit. In both cases, a much higher electromagnetic field intensity is needed than the field intensity normally used to detect the presence of a label. The maximum energy is transmitted to the resonant frequency of the label. Devices to realize this are known by the name of "deactivators". A deactivator of the above kind is disclosed in U.S. Pat. No. 4,567,473. The known deactivator comprises means for generating, in a limited region, an electromagnetic field with a frequency which, to a certain extent, is varied around the resonant frequency of the labels. Furthermore, means are provided in the deactivator to detect the presence of a label in the region, as well as the resonant frequency of the label. When the deactivator has determined the presence and the resonant frequency of a label, such a strong field is generated with the resonant frequency in question that the resonant properties of the label are permanently disabled. The detection system is also active during the increased field intensity, and as soon as the label is no longer detected, and so the resonant circuit has been disabled, the field intensity is returned to the normal detection level. In an alternative method, also described in U.S. Pat. No. 4,567,473, use is made of a field with an increased field strength and a swept frequency. One disadvantage of this is that the bandwidth of a label is generally about 10% of the frequency sweep. During about 90% of the deactivating period, therefore, a large field is generated unnecessarily, which involves a number of disadvantages. Thus, for example, the operator of the deactivator is subjected to a relatively high biological load with a high-frequency electromagnetic field. Also, the prior apparatus has an unnecessarily high power consumption. Furthermore, both existing methods often result in spurious radiation as a result of the strong non-synchronized deactivating fields, which may cause interference with interrogation fields generated elsewhere, resulting in false alarm or a reduced chance of detection of the labels.